A conventional loudspeaker utilizes a round-shaped electromagnetic transducer to drive a cone-type membrane to radiate sound. In general, an additional enclosure is necessary to facilitate sound radiation, which makes the loudspeaker cumbersome, weighty and having dead corner for sound radiation, etc. Recently, flat display and mobile communication devices such as notebook, cellular phone and personal digital assistant (PDA), are rapidly developed toward miniaturization. The integration of transparent panel-form loudspeakers with the flat display and mobile communication devices can greatly enhance the performance of such devices. Therefore, this conventional loudspeaker is gradually replaced by a panel-form loudspeaker.
FIGS. 1(a) and 1(b) are a top view and a cross-sectional view of a traditional panel-form loudspeaker, respectively. This panel-form loudspeaker radiates sound by exciting a radiating panel to produce flexural vibration. Such panel-form loudspeaker 1 comprises a frame 11, a radiating panel 12, an electromagnetic transducer 13 and a suspending unit 14. The frame 11 is in a rectangular shape with a hollow region in the center thereof. The cross section of the frame 11 is substantially L-shaped. The horizontal and the vertical portion of the L-shaped cross section are referred hereinafter as a bottom portion and a peripheral portion, respectively. The suspending unit 14 are attached onto and supported by the bottom portion of the frame 11. The radiating panel 12 is positioned by the peripheral portion of the frame 11. The suspending unit 14 comprises a plurality of separate strips. These strips can be selected from rubber-impregnated strips, foam type continuous strips and corrugated shell strips.
The transducer 13 is attached to the bottom surface of the radiating panel 12 and principally comprises a magnet unit and a voice coil unit. The magnet unit comprises a disk-shaped top plate 131, a cylindrical permanent magnet 132 and a permeance unit 133 such that a magnetic field is generated in a gap therebetween. The voice coil unit comprises a contact sheet 134 and a coil 135. Thus, when electric current flows through the coil 135, the voice coil unit will generate a motion in a direction vertical to the magnetic field so as to excite the radiating panel 12 to generate flexural vibration and radiate sound. The resilience support 136 is employed to fix the voice coil unit to be immersed in the magnetic field between the top plate 131, the permanent magnet 132 and the permeance unit 133. In general, the resilience support 136 also works as a damper to suppress undesirable vibrations of the radiating panel.
The transducer 13 is usually arranged at a specified location of the radiating panel to produce an effective modal vibration. Since the arrangement of resilience support 136 facilitates increasing rigidity of the radiating panel, some undesirable effects occur. For example, a relatively higher initial response frequency and considerable fluctuations are found on the sound pressure spectrum over the audible frequency range. In addition, when input power is augmented, a more apparent non-linear relation exists between the pressure response and the power. Please refer to FIG. 1(b). As known, the increasing area of the resilience support 136 facilitates overcoming the above-mentioned problems. However, the increasing area requires a larger permeance unit 133 because the resilience support 136 is coupled to the permeance unit 133. Due to the larger size of the permeance unit 133, the cost, the magnetic loss and the volume of the overall transducer will be inevitably increased.
The process for assembling the above panel-form loudspeaker will be described in brief as follows. Firstly, the strips of the suspending unit 14 are attached onto the bottom portion of the frame 11. Then, the transducer 13 is attached to a bottom surface of the radiating panel 12 at a specified location thereof. Afterwards, the radiating panel 12 is attached onto the suspending unit 14 so as to assemble the panel-form loudspeaker.
The above-mentioned assembling process has some problems. For example, it is time-consuming when a large number of strips are used. Some strips may be deviated from their designed locations, which impairs uniformity of the product. Moreover, the step of attaching the transducer 13 onto the bottom surface of the radiating panel 12 should be performed mechanically to achieve precise alignment at the specified location. The cost associated to the precise alignment will be increased if the dimension of the radiating panel changes.